A capacitor which contains a conventional polarized electrode is described in, for example, Japanese Patent Unexamined Publication No. H11-154630, Japanese Patent Unexamined Publication No. 2004-186266, etc.
The structure of a conventional electrochemical capacitor 10 is described referring to FIG. 6. Capacitor element 11 shown in FIG. 6 includes a polarized electrode which has been formed of current collector 12 made of an aluminum foil or the like provided with an anchor coat layer formed at both surfaces, and electrode sheet 13 attached on the anchor coat layer. Capacitor element 11 is formed of two sheets of the polarized electrodes wound together with separator 14 disposed in between the two sheets.
Electrochemical capacitor 10 further includes a pair of aluminum lead wires 15 connected with respective polarized electrodes, metal case 16 made of aluminum which houses capacitor element 11 together with an electrolyte for driving (not shown), and sealing rubber 17 for sealing the open end of metal case 16, which sealing rubber is provided with through holes for allowing the pair of lead wires 15 to be led out and fitted to open end metal case 16 and then the edge of metal case is machined.
FIG. 7 is a perspective view used to describe the structure of polarized electrode 19, which forms capacitor element 11. FIG. 8 is a cross sectional view of the polarized electrode. As shown in FIG. 7 and FIG. 8, polarized electrode 19 is consisting of current collector 12 made of an aluminum foil, etc. whose surface has been roughened, anchor coat layer 18 provided at both surfaces of current collector 12, and electrode sheet 13 attached on anchor coat layer 18. Anchor coat layer 18 is formed of a conductive carbon of graphite or carbon black, and a binder.
Reason why it uses electrode sheet 13 is explained as followings. If an electrode layer is formed by coating a paste of electrode material on current collector 12 via anchor coat layer 18, a binder contained in anchor coat layer 18 may be decomposed and the decomposed binder may dissolve anchor coat layer 18. In order to avoid such a phenomenon to happen, electrode sheet 13 is provided beforehand, by kneading an activated carbon, a conductivity-providing material and a binder, and then crushing the above kneaded substance to be formed into a sheet form. Electrode sheet 13 thus provided is attached on anchor coat layer 18.
The above-configured conventional electrochemical capacitor exhibits a high power density and provides a large current in a moment, as compared to secondary storage batteries. So, it is being studied to use the capacitors as a power assist of the electric mobiles, and for the uninterruptible power systems, etc. The capacitors are requested to exhibit a still higher power density, as well as to improve the energy density, the reliability under a continuous voltage application and the durability against repeated cycles of charge-discharge operation.
The most effective means for increasing the energy density and the power density of the above described electrochemical capacitors is thinning the current collector. The thinner current collector brings about remarkable advantages; an increased electrical capacitance which is the reflection of an increased volume shared by the electrode layer in a cell, and a reduced internal resistance which is the reflection of an increased opposing area between the polarized electrodes.
In order to reduce the internal resistance and to curtail a deterioration of the electrical capacitance, conventional electrochemical capacitors use a surface-etched and roughened aluminum foil for the current collector. However, the surface-etched and roughened aluminum foil has a problem; that is, strength of the foil is not high enough to withstand a tension it could encounter during manufacturing stage, sometimes it is broken during manufacturing. Thus, it is not easy to make the foil thinner.
On the other hand, an aluminum foil whose surface is not roughened (hereinafter referred to as a plain foil) has a higher strength, so it can be made thinner. However, a plain foil, or an un-etched foil, has no etch pit. So, a contact area between electrode layer and current collector decreases and the coupling strength deteriorates. As the result, the internal resistance increases and the reliability deteriorates with an electrochemical capacitor.
Furthermore, it is difficult in the conventional capacitors to form an electrode layer directly on anchor coat layer 18. Therefore, electrode sheet 13 is employed. An additional step for manufacturing electrode sheet 13 brings about a lowered productivity, generating a serious cost issue.